i) Field of the Invention
The disclosed invention pertains to absorbent fibres made from regenerated low-substituted alkali metal carboxymethyl cellulose using a wet-spinning process. The fibres have high liquid absorption and high liquid retention capacities and are suitable for the use in wound dressing, absorbent hygiene products, wipes and medical applications.
ii) Description of the Prior Art
Absorbent polysaccharide fibres used in wound dressing, surgical dressing, surgical sponges, infant diapers, adult incontinence products, catamenial napkins, tampons, etc. are well known in the art. Polysaccharide fibres are derived from renewable resources and examples include: regenerated and non-regenerated cellulose fibres, chemically modified cellulose fibres, alginate fibres, chitosan fibres, pectin fibres, hyaluronic acid fibres and other fibres derived from polysaccharides or gums. Absorbent fibres and superabsorbents—materials capable of absorbing more than ten times their own weight of liquids, bodily fluids or blood—are used to enhance liquid absorbency as well as to ensure high liquid retention.
One polysaccharide used to a great extent to enhance liquid absorbency is sodium carboxymethyl cellulose (CMC). Besides being one of the most important cellulose derivatives, it is also relatively inexpensive. Carboxymethyl cellulose is chemically modified cellulose with the carboxymethyl group as a substituent. The carboxymethyl group contains a carboxyl group that can greatly improve the hydrophilic behaviour of cellulose. The properties and performance of carboxymethyl cellulose depend on the number of present carboxyl groups, i.e. the degree of substitution (DS).
Absorbent carboxymethyl cellulose fibres are highly desirable in hygiene and medical applications. However, the known processes for producing such fibres are complex and expensive: (i) spinning carboxymethyl cellulose fibres is difficult; (ii) carboxymethylation of regenerated or non-regenerated fibres incurs additional cost; (iii) carboxymethyl cellulose added into a spinning dope before fibres are made may slow down production; (iv) fibres from carboxymethyl cellulose having high DS are difficult to regenerate; and (v) fibres from carboxymethyl cellulose having low DS are difficult to process.
U.S. Pat. No. 2,495,767 describes continuous fibres spun from CMC having a DS between 0.5 and 1.0. The spinning dope contains 5% carboxymethyl cellulose and is extruded through a spinneret into a coagulation bath containing polyvalent metal cations such as copper, iron, lead, chromium, aluminium or their combination. The linear density of the spun fibres is between 72-566 dtex. Absorbent fibres can be also produced by extruding CMC solution through a spinneret into a coagulation bath that contains a water-miscible organic solvent and a cross-linker as suggested in U.S. Pat. No. 6,846,924. The patent claims that CMC with a DS below 0.35 has poor liquid absorption properties, and therefore, CMC having a DS above 0.35 is required. Water-miscible organic solvents, preferably alcohols, such as methanol, ethanol or isopropanol, or ketones, such as acetone, are used to regenerate fibres in the coagulation bath. Polyelectrolytes, such as polyvinyl amine and other quaternary polyamines or polyvalent metal cations, such as calcium, zirconium, magnesium, iron or aluminium, are used as cross-linking agents. The fibres are spun through jets with a diameter of 200 μm from a dope that contains between 7-12 CMC, depending on degree of polymerization. The saline (0.9% NaCl) absorption of the CMC fibres is 30 g/g.
Carboxymethyl cellulose is water soluble, however it can be cross-linked to obtain water-insoluble CMC. U.S. Pat. No. 3,589,364 relates to bibulous fibres produced by wet cross-linking of swollen regenerated or non-regenerated cellulose fibres with epichlorohydrin, followed by carboxymethylation. The degree of substitution is between 0.4-1.6, yet the carboxymethylated fibres are water-insoluble due to cross-linking. The total saline (0.9% NaCl) absorbency is between 4-12 g/g, while water absorbency is between 9-33 g/g. It is also possible to perform cross-linking and carboxymethylation at the same time as revealed in U.S. Pat. No. 4,068,068; U.S. Pat. No. 4,187,342; and U.S. Pat. No. 4,248,595.
CA Pat. No. 1,062,425 describes hydrophilic shaped structures of water-insoluble cellulose ethers that are capable of forming fibres or films. Hydroxyethyl cellulose, methyl-hydroxyethyl cellulose or carboxymethyl cellulose can be cross-linked to a low degree before its addition to a dope. The maximum attainable loading of pre-cross-linked CMC is 50% while the maximum water absorbency is 4.4 g/g. It is also possible to surface treat fibres or films during formation with pre-cross-linked cellulose ethers.
Besides cross-linking, CMC can be made substantially water insoluble by changing conditions during etherification reaction. U.S. Pat. No. 3,678,031 refers to substantially insoluble alkali metal carboxymethyl cellulose prepared with a molar excess of the neutralized etherifying agent, with extended reaction time from 3 hours to 24 hours and elevated temperature from 60° C. to 80° C. The substantially insoluble CMC has a DS between 0.4-1.2 and can absorb up to 70 times its own weight of aqueous solution. U.S. Pat. No. 3,723,413 describes water-insoluble carboxymethyl cellulose with a DS between 0.35 and 1.4 which is prepared by removing a portion of carboxymethylating reactants, residues and impurities formed during the reaction and then heat-treating the carboxymethyl cellulose. Based on the presented results, the calculated water absorbency of the resulting water-insoluble CMC is 6.2 g/g.
U.S. Pat. No. 4,256,111 describes a process where cellulose fibres, chemically modified by either phosphorylation or carboxymethylation, are refined to produce microfibrils. The suspension of microfibrillated cellulose is centrifuged to remove unbound water and the resultant viscous suspension of filament is then extruded in the form of continuous fibres using solvent exchange and subsequent drying. The fibre diameter is relatively large between 127-889 μm. Carboxymethylated fibres have total water absorbency between 40-50 g/g.
Polysaccharide fibres from the mixture of sodium alginate and sodium carboxymethyl cellulose can be regenerated in a coagulation bath containing a solution of calcium chloride as described in U.S. Pat. No. 7,229,689. The content of CMC is up to 15% and the dope is extruded through a 40,000-jet spinneret. The diameter of the jets is 70 μm. The fibres are carded and needled-punched to produce a non-woven fabric that absorbs around 21 g of saline (0.9% NaCl)/g fibre. In U.S. Pat. No. 6,080,420 the amount of carboxymethyl cellulose is increased to 30%. U.S. Pat. No. 6,140,257 relates to alginate/CMC fibres composed of at least of 10% alginate but less than 50% by weight. The remainder of the fibre composition is another polysaccharide, such as carboxymethyl cellulose, where the content of the other polysaccharide is between 40-90%. In addition, pectin could also be added from 0-20%. A 400-jet spinneret head is used to extrude the dope into a coagulation bath containing calcium chloride.
U.S. Pat. No. 20120209234 describes collapsible, hollow, rayon fibres that contained 5-50% CMC having a DS of 0.6-1.2. The absorbency of the resulting fibres is 5.2 g/g. The disadvantage of using CMC in rayon process is that the fibres tend to stick together due to gel formation at the surface of the fibres, and above 15% CMC content in the dope, the fibres do not float as conventional rayon fibres do, but sink which hinders production. U.S. Pat. No. 4,199,367 and U.S. Pat. No. 4,289,824 describe rayon staple fibres that contain 10-40% CMC having high fluid holding capacity. Carboxymethyl cellulose is added to viscose dope before the fibres are spun and the resulting alloy fibres have a maximum retention of 7 g/g. U.S. Pat. No. 3,423,167 relates to rayon fibres with a CMC content between 1-50% which have been subjected to wet-cross-linking. Cross-linking of swollen fibres in wet state leads to high water imbibition as opposed to cross-linking of dry collapsed fibres which have low liquid absorption capacity. U.S. Pat. No. 6,548,730 refers to carboxymethylation of regenerated cellulose fibres with sodium hydroxide and monochloroacetic acid. The carboxymethyl groups are believed to be predominantly in the amorphous regions and the degree of substitution is between 0.12-0.35.
WO Pat. No. 1993012275 describes solvent-spun fibres from tertiary N-oxide (referred to as Lyocell, also known as Tencel) that are carboxymethylated to obtain a DS between 0.2-0.5. The carboxymethylation is performed in a post-treatment of regenerated fibres with sodium hydroxide and monochloroacetate agent. The fibres are capable of absorbing 8-15 g saline (0.9 NaCl)/g fibres. Similarly, WO Pat. No. 1994016746 relates to wound dressing made of carboxymethyl cellulose fibres produced in the reaction of Lyocell/Tencel with sodium hydroxide and sodium monochloroacetate. The resulting fibres have a DS between 0.25-0.45 and absorbency of saline solution (0.9% NaCl) between 15-25 times their own weight.
U.S. Pat. No. 20100144669 describes preparation of fibres from carboxymethyl cellulose that was dissolved in an ionic liquid consisting of a tertiary amine N-oxide.
Hydroxyethyl cellulose (HEC) is a cellulose ether that is similar in behaviour to carboxymethyl cellulose. HEC can be spun using conventional rayon spinneret as described in Dongmei Li at al., J. Appl. Polym. Sci., 117, 767-774 (2010). Low-substituted HEC having a DS of 0.49 is dissolved in sodium hydroxide and then regenerated in a coagulation bath containing sulphuric acid, sodium sulphate and zinc sulphate. The HEC concentration is 7.6%, while NaOH concentration is 8%. The resulting HEC fibres have a titre of 1.67 dtex.
As seen from above, carboxymethylated fibres can be obtained by various methods including: (i) spinning CMC directly to a coagulation bath containing polyvalent ions as cross-linkers which may, or may not, also contain polyelectrolytes and organic solvents; (ii) admixing CMC to conventional rayon or Lyocell/Tencel fibres; (iii) co-spinning CMC with another polysaccharide such as alginate that is easier to regenerate; (iv) pre-cross-linking CMC prior to its addition to a dope containing other polysaccharides; (v) carboxymethylating conventional rayon or Lyocell/Tencel fibres; (vi) heat-treating CMC; (vii) spinning CMC from ionic liquids; and (viii) spinning microfibrillated carboxymethylated cellulose. In cases where carboxymethyl cellulose is used in a dope, the degree of substitution is above 0.35, preferably 0.4-1.2. In cases where regenerated or non-regenerated fibres are carboxymethylated, the degree of substitution is ranging from 0.12-0.5. In the post-treated fibres, carboxymethyl groups are likely to be located in the amorphous regions of the fibres and at or near the fibre surface because the carboxymethylation times are not long enough to permit even distribution of carboxymethyl groups inside the fibre. Alloy fibres spun from a dope containing high-substituted CMC consist of highly charged CMC molecules surrounded by underivatized, uncharged cellulose molecules, and may not be particularly absorbent.